[SystemZ][z/OS] Fix handling of dirs with filesystem tests

The aim of this patch is to fix the post processing that is happening on the temporary test directories upon scope exit. In particular, ~scoped_test_env aims to chmod and remove the temporary directories; however,

bad symlinks are followed and we get "No such file or directory". FIX: use find as alternative to chmod and avoid -follow option.
Attempting to remove read-only files on z/OS prompts a message asking for confirmation. FIX: use the -f option to delete read-only files immediately without asking for confirmation.
Some libcxx tests such as libcxx/test/std/input.output/filesystems/cl ass.directory_entry/directory_entry.cons/path.pass.cpp set the dir permissions to none. In turn, recursively doing chmod (-R) does not set the file permissions needed to be able to remove the dir on z/OS only. FIX: use find as alternative to chmod -R, which does not run into this issue on z/OS.

Differential Revision: https://reviews.llvm.org/D108326